Advertisement

Advertisement

New Scientist Live

Milky Way feels tug of largest mass in the universe

By Maggie McKee

This image, taken with the European Southern Observatory’s Wide Field Imager, shows the galaxies visible in the “Zone of Avoidance” in the direction of the “Great Attractor”

(Image: ESO)

The Milky Way is one of a long line of galaxies streaming towards a point about 500 million light years away, reveals a new X-ray survey. The work confirms previous studies showing the Milky Way is not drawn towards a concentration of mass called the Great Attractor – but to an even more massive region behind it.

The study is the first to search for the X-ray glow of massive galaxy clusters that lie in the plane of the Milky Way, where dust obscures observations at visible wavelengths. This plane covers a third of the sky and blocks much of the visible light from objects behind it, earning it the title the Zone of Avoidance.

But about 20 years ago, intriguing observations at a range of wavelengths suggested that the Milky Way was being pulled toward a concentration of mass in the zone&colon; the Great Attractor. It was estimated to contain more than 10 million billion times the mass of the Sun and lie about 140 million light years away.

Advertisement

The possibility that such a massive entity was located relatively near the Milky Way suggested there were many such “overdensities” and that the universe therefore contained more matter than expected, says Dale Kocevski, a graduate student at the University of Hawaii in Honolulu, US, and part of the new X-ray survey.

Some subsequent observations at infrared wavelengths suggested the Milky Way was in fact not drawn towards the Great Attractor. But Kocevski says many of these studies were based on observations of individual galaxies, which cannot be seen at distances beyond about 200 million light years.

Telltale glow

The latest survey, called Clusters in the Zone of Avoidance (CIZA), has searched about four times as far, looking for the telltale X-ray glow of hot gas in groupings of galaxies. The survey searches for cluster candidates in archived data from the ROSAT X-ray satellite, which flew in the 1990s. Then it observes the faint optical signal from these candidates to determine their redshift, or distance.

So far it has identified 205 galaxy clusters, 79% of which are new discoveries. In a paper submitted to the Astrophysical Journal, the CIZA team reports that the Great Attractor has about a tenth the mass that was originally attributed to it. “We ended up finding out the Great Attractor wasn’t that great,” Kocevski told New Scientist.

Instead, the Milky Way is part of a surge of galaxies moving past the Great Attractor, towards the Shapley supercluster, an agglomeration of 17 clusters of galaxies about 490 million light years away.

It is the most massive known structure in the observable universe, topping the list of 220 known superclusters, and is four times as massive as the Great Attractor. But Kocevski says it is possible there may be something yet more massive behind Shapley that is simply not detectable.

He says Shapley’s extreme distance suggests such concentrations of mass are actually not so common in the universe, agreeing with other estimates of the universe’s density. He adds that the research is also important because it shows “things that far away really do seem to affect our dynamics”.

Dark matter

Michael Rowan-Robinson, an astronomer at Imperial College London, UK, agrees. “This study will certainly contribute to our understanding of our galaxy’s motion with respect to the cosmic frame,” he told New Scientist. “Modelling this motion can give us an independent estimate of the dark matter density in the universe.”

Rowan-Robinson’s own earlier studies also argued against the influence of the Great Attractor.

But Ofer Lahav, an astrophysicist at University College London, UK, disagrees. He points out that cluster surveys like CIZA tend to emphasise Shapley’s predominance, while surveys of individual galaxies show the Milky Way and its neighbours are being pulled between the Great Attractor and another supercluster on the opposite part of the sky.

“The question is, which are better tracers [of matter]&colon; the galaxies or the clusters?” he asks. “In my view, galaxies trace it better,” he told New Scientist, adding that clusters only represent the densest “peaks” in the distribution of matter through space.

The CIZA survey also discovered the first supercluster completely hidden by the Milky Way’s veil of dust – a grouping of eight galaxy clusters, lined up in the shape of a “T”, that lie about 500 million light years away.